The Universal Serial Bus (USB) is a relatively easy to use interface for connecting peripheral devices to a host system. In general when a peripheral device is connected to a host by a USB, the host will auto-detect and auto-configure the device.
The initial type of USB connection which achieved widespread use (referred to as USB 1.1) is defined by the “universal Serial Bus Revision 1.1 specifications. An improved USB which has also gained widespread use (referred to as USB 2.0) is defined by the “Universal Serial Bus Revision 2.0 specification”. The specification for both the 1.1 and 2.0 USB interfaces are publicly available and they are incorporated herein by reference.
A USB port may support either one of two power levels. The first power level has a maximum current of 100 ma. The second power level has a maximum current level of 500 ma. Many peripheral devices can operate satisfactorily at the 100 ma power level and other devices can operate satisfactorily at the 500 ma power level. A device that requires more than 100 ma and less than 500 ma must be plugged into a USB port that supports the 500 ma power level. According to the USB specifications, any device that requires more than 500 ma of current must have an own external power supply or batteries.
Among the disadvantages of using an external power supply or batteries are the size and weight of such devices.